Pneumatic tire and crosslinked rubber composition

ABSTRACT

A pneumatic tire includes bead cores, a carcass ply, an inner liner and the tread, and the crosslinked rubber composition, of the present invention, wherein the tread and the crosslinked rubber composition have a volume of the low density region of 35% or more at elongation by an applied stress of 1.5 MPa, a volume of the void portion of 7.5 or less at elongation by an applied stress of 3.0 MPa and a ratio of 40% by mass or more of a component having a weight-average molecular weight of not less than 1,000,000 in a molecular weight distribution measured by gel permeation chromatography are excellent in abrasion resistance.

TECHNICAL FIELD

The present invention relates to a pneumatic tire and a crosslinkedrubber composition.

BACKGROUND OF THE INVENTION

It is known that generally a crosslinked rubber composition used for atire tread is excellent in breaking resistance and abrasion resistanceas its average molecular weight becomes higher (for example, PatentDocument 1, etc.). One of factors thereof is thought to be such that asthe average molecular weight becomes higher, the number of terminals ofa rubber component which can become a starting point of breakagedecreases.

However, a higher average molecular weight does not necessarily lead toexcellent breaking resistance and abrasion resistance, and there is anexception. Namely, sometimes there is a phenomenon such that though anaverage molecular weight of a crosslinked rubber composition is high,good results cannot be obtained from measurement of abrasion resistanceof this crosslinked rubber composition and a rubber product thereof.

In order to make a mechanism of causing such an exception clear,breakage and friction phenomena have been observed by means of variousmethods. However, the mechanism has not been made clear completely.

Meanwhile, an X-ray computerized tomography is known as a technology foranalyzing a material constituting a solid sample and a low densityregion contained in an inside thereof. For example, Patent Document 2describes a method of visually analyzing a material constituting afriction member and a low density region contained inside thereof.However, analyzing a sample material in a state, such as elongation, ofbeing applied with an external energy or analyzing a density of thematerial is not described in Patent Document 2.

PATENT DOCUMENTS

Patent Document 1: JP 2013-032497 A

Patent Document 2: JP 2009-085732 A

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

An object of the present invention is to provide a pneumatic tire beingexcellent in abrasion resistance and a crosslinked rubber composition.

Means to Solve the Problem

The inventors of the present invention have made intensive studies, andas a result, have found that growth of cracking generated inside acrosslinked rubber composition can be inhibited and abrasion resistancecan be enhanced more by increasing a low density region at elongation byan applied stress of 1.5 MPa, decreasing a void portion at elongation byan applied stress of 3.0 MPa and increasing a ratio of a componenthaving a weight-average molecular weight of not less than 1,000,000, andthus have completed the present invention.

Namely, the present invention relates to a pneumatic tire comprisingbead cores provided on a pair of right and left bead portions,respectively, a carcass ply extending from a crown portion to the bothbead portions through both side wall portions and moored to the beadcores, an inner liner disposed at an inner side than the carcass ply ina direction of a tire diameter, and a tread disposed at an outer sidethan the carcass ply in a direction of a tire diameter and having avolume of the low density region of 35% or more at elongation by anapplied stress of 1.5 MPa, a volume of the void portion of 7.5% or lessat elongation by an applied stress of 3.0 MPa and a ratio of 40% by massor more of a component having a weight-average molecular weight of notless than 1,000,000 in a molecular weight distribution measured by gelpermeation chromatography.

Further, the present invention relates to a crosslinked rubbercomposition having a volume of the low density region of 35% or more atelongation by an applied stress of 1.5 MPa, a volume of the void portionof 7.5% or less at elongation by an applied stress of 3.0 MPa and aratio of 40% by mass or more of a component having a weight-averagemolecular weight of not less than 1,000,000 in a molecular weightdistribution measured by gel permeation chromatography.

It is preferable that the rubber component is a rubber componentcomprising one or more of rubber components comprising a conjugateddiene compound.

It is preferable that the above-mentioned low density region is a regionhaving a density of 0.1 to 0.8 time the density of a crosslinked rubbercomposition before the elongation.

It is preferable that the above-mentioned void portion is a regionhaving a density of 0 to 0.1 time the density of a crosslinked rubbercomposition before the elongation.

It is preferable that a method of evaluating volumes of the low densityregion and the void portion is an X-ray computerized tomography.

It is preferable that a decay time of a phosphor for converting theX-ray into a visible light is 100 ms or less.

It is preferable that a luminance of the X-ray is 10¹⁰photons/s/mrad²/mm²/0.1% bw or more.

Effect of the Invention

According to the pneumatic tire of the present invention comprising atread having a large volume of the low density region at elongation byan applied stress of 1.5 MPa, a small volume of the void portion atelongation by an applied stress of 3.0 MPa and a high ratio of acomponent having a weight-average molecular weight of not less than1,000,000 in a molecular weight distribution measured by gel permeationchromatography (GPC), and to the crosslinked rubber composition of thepresent invention having a large volume of the low density region atelongation by an applied stress of 1.5 MPa, a small volume of the voidportion at elongation by an applied stress of 3.0 MPa and a high ratioof a component having a weight-average molecular weight of not less than1,000,000 in a molecular weight distribution measured by gel permeationchromatography (GPC), it is possible to provide a pneumatic tire and acrosslinked rubber composition being excellent in abrasion resistance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of an example of an evaluationdevice for evaluating a density distribution of a crosslinked rubbercomposition at elongation thereof.

FIG. 2 is a flow chart showing steps of procedure of an evaluationmethod of a density distribution of a crosslinked rubber composition atextension thereof.

EMBODIMENT FOR CARRYING OUT THE INVENTION

The crosslinked rubber composition of the present invention is acrosslinked rubber composition having a large volume of the low densityregion at elongation by an applied stress of 1.5 MPa, a small volume ofthe void portion at elongation by an applied stress of 3.0 MPa and ahigh ratio of a component having a weight-average molecular weight ofnot less than 1,000,000 in a molecular weight distribution measured bygel permeation chromatography (GPC). It is noted that the crosslinkedrubber composition as used herein is a rubber composition subjected tocrosslinking using a vulcanizing agent and an organic peroxide.

Rubber Component

The rubber component is not limited particularly, and among rubbercomponents including diene rubbers such as isoprene rubber, butadienerubber (BR), styrene-butadiene rubber (SBR), styrene-isoprene-butadienerubber (SIBR), chloroprene rubber (CR) and acrylonitrile-butadienerubber (NBR), and butyl rubber, which have been used in a rubberindustry, one or more thereof can be appropriately selected and used. Inparticular, it is preferable that the rubber component comprises one ormore of rubber components comprising a conjugated diene compound, and arubber component comprising SBR and BR is preferable from the viewpointof a balance of fuel efficiency, abrasion resistance, durability and wetgrip performance.

SBR is not limited particularly, and examples thereof include anemulsion-polymerized SBR (E-SBR), a solution-polymerized SBR (S-SBR) andthe like. The SBR may be oil-extended or may not be oil-extended.Further, a terminal-modified S-SBR and a main chain-modified S-SBR whichhave enhanced capability to interact with a filler can also be used.These SBRs may be used alone or may be used in combination of two ormore thereof.

A styrene content of the SBR is preferably not less than 16% by mass,more preferably not less than 20% by mass, further preferably not lessthan 25% by mass, particularly preferably not less than 30% by mass,from the viewpoint of grip performance. When the styrene content is toolarge, styrene groups become in proximity to each other, a polymerbecomes too hard and crosslinking becomes non-uniform, which maydeteriorate blowing property during running at high temperature, andfurther there is a tendency that since temperature dependency of theperformances is increased and the performances can be changed largelywith respect to a temperature change, stable grip performance cannot beobtained at a middle/latter stage of running. Therefore, the styrenecontent is preferably not more than 60% by mass, more preferably notmore than 50% by mass, further preferably not more than 40% by mass. Itis noted that the styrene content of the SBR as used herein iscalculated in accordance with ¹H-NMR measurement.

A vinyl content of the SBR is preferably not less than 10%, morepreferably not less than 15%, from the viewpoint of Hs of thecrosslinked rubber composition and grip performance. On the other hand,from the viewpoint of grip performance, EB (durability) and abrasionresistance, the vinyl content of the SBR is preferably not more than90%, more preferably not more than 80%, further preferably not more than70%, particularly preferably not more than 60%. It is noted that thevinyl content of the SBR (an amount of 1,2-bond butadiene unit) as usedherein can be determined by an infrared absorption spectrum analysismethod.

A glass transition temperature (Tg) of the SBR is preferably not lowerthan −45° C., more preferably not lower than −40° C. The Tg ispreferably not higher than 10° C., and the Tg is more preferably nothigher than 5° C. from the viewpoint of prevention of a crack due toembrittlement during a winter season in the Temperate Zone. It is notedthat a glass-transition temperature of the SBR as used herein is a valuemeasured by conducting a differential scanning calorimetry measurement(DSC) under the condition of a temperature elevation rate of 10°C./minute in accordance with JIS K 7121.

A weight-average molecular weight (Mw) of the SBR is preferably not lessthan 700,000, more preferably not less than 900,000, further preferablynot less than 1,000,000 from the viewpoint of grip performance andblowing property. On the other hand, the weight-average molecular weightis preferably not more than 2,000,000, more preferably not more than1,800,000 from the viewpoint of blowing property. It is noted that theweight-average molecular weight of the SBR as used herein can becalibrated with standard polystyrene based on measurement valuesdetermined with a gel permeation chromatography (GPC) (GPC-8000 seriesmanufactured by Tosoh Corporation; detector: differential refractometer;column: TSKGEL SUPERMALTPORE HZ-M manufactured by Tosoh Corporation).

An SBR content in the rubber component is preferably not less than 30%by mass, more preferably not less than 40% by mass, for the reason thata sufficient grip performance can be obtained. On the other hand, theSBR content is preferably not more than 90% by mass, more preferably notmore than 85% by mass, further preferably not more than 80% by mass,from the viewpoint of abrasion resistance, grip performance and fuelefficiency.

Particularly for the reason that higher grip performance and blowingproperty can be exhibited, it is preferable that the rubber componentcomprises 40% by mass or more of SBR having a styrene content of 16 to60% by mass, and it is more preferable that the rubber componentcomprises 50% by mass or more of SBR having a styrene content of 25 to55% by mass.

The BR is not limited particularly, and for example, BRs (high-cis BRs)having a high-cis content such as BR1220 available from ZEONCORPORATION, and BR130B and BR150B available from Ube Industries, Ltd.;modified BRs such as BR1250H available from ZEON CORPORATION; BRs havingsyndiotactic polybutadiene crystal such as VCR412 and VCR617 availablefrom Ube Industries, Ltd.; BRs (rare-earth BRs) synthesized using arare-earth element catalyst such as BUNA-CB25 available from LANXESSJapan, and the like can be used. These BRs may be used alone or may beused in combination of two or more thereof. Particularly high-cis BRsand rare-earth BRs are preferable from the viewpoint of processabilityand excellent abrasion resistance and breaking resistance.

When the rubber component comprises the BR, a content of the BR in therubber component is preferably not less than 10% by mass, morepreferably not less than 15% by mass, further preferably not less than20% by mass, from the viewpoint of abrasion resistance, grip performanceand fuel efficiency. Further, the content of the BR is preferably notmore than 70% by mass, more preferably not more than 60% by mass, fromthe viewpoint of abrasion resistance, grip performance and fuelefficiency.

The above-mentioned filler can be optionally selected from those whichhave been usually used in a crosslinked rubber composition, and carbonblack and silica are preferable.

Examples of the carbon black include furnace black, acetylene black,thermal black, channel black, graphite and the like and these carbonblacks may be used alone or may be used in combination with two or morethereof. Among them, furnace black is preferable for the reason that lowtemperature characteristics and abrasion resistance can be enhanced ingood balance.

A nitrogen adsorption specific surface area (N₂SA) of the carbon blackis preferably not less than 70 m²/g, more preferably not less than 90m²/g, from the viewpoint that sufficient reinforcing property andabrasion resistance can be obtained. Further, the N₂SA of the carbonblack is preferably not more than 300 m²/g, more preferably not morethan 250 m²/g, from the viewpoint of excellent dispersibility and aproperty of being hard to generate heat. It is noted that the N₂SA ofthe carbon black as used herein is measured in accordance with JISK6217-2 “Carbon black for rubber industry—Fundamentalcharacteristics—Part 2: Determination of specific surface area—Nitrogenadsorption method—Single-point procedures”.

When the rubber composition comprises the carbon black, the contentthereof is preferably not less than 3 parts by mass, more preferably notless than 4 parts by mass, based on 100 parts by mass of the rubbercomponent. When the content is less than 3 parts by mass, there is atendency that a sufficient reinforcing property cannot be obtained. Onthe other hand, the content of the carbon black is preferably not morethan 200 parts by mass, more preferably not more than 150 parts by mass,further preferably not more than 60 parts by mass. When the content ismore than 200 parts by mass, there is a tendency that processability islowered, heat generation is prone to arise and abrasion resistance islowered.

Silica is not limited particularly, and there are, for example, silicaprepared by a dry method (anhydrous silica) and silica prepared by a wetmethod (hydrous silica), and hydrous silica is preferred for the reasonthat many silanol groups are contained.

A nitrogen adsorption specific surface area (N₂SA) of the silica ispreferably not less than 80 m²/g, more preferably not less than 100m²/g, from the viewpoint of durability and an elongation at break. Onthe other hand, the N₂SA of the silica is preferably not more than 250m²/g, more preferably not more than 220 m²/g, from the viewpoint of fuelefficiency and processability. It is noted that the N₂SA of the silicaas used herein is a value measured in accordance with ASTM D3037-93.

When the rubber composition comprises the silica, the content of thesilica is preferably not less than 5 parts by mass, more preferably notless than 10 parts by mass based on 100 parts by mass of the rubbercomponent, from the viewpoint of durability and an elongation at break.On the other hand, the content of the silica is preferably not more than200 parts by mass, more preferably not more than 150 parts by mass, fromthe viewpoint of enhancing dispersibility during kneading and forinhibiting lowering of processability due to re-agglomeration of silicaduring heating at rolling and during storage after rolling.

When the rubber composition comprises the silica, it is preferable thatthe silica is used in combination with a silane coupling agent. Anysilane coupling agent which has been used in combination with silica inthe rubber industry can be used as the silane coupling agent, andexamples thereof include sulfide silane coupling agents such as Si75,Si266 (bis(3-triethoxysilylpropyl)disulfide) manufactured by EvonikDegussa and Si69 (bis(3-triethoxysilylpropyl)tetrasulfide) manufacturedby Evonik Degussa; mercapto silane coupling agents (mercaptogroup-containing silane coupling agents) such as3-mercaptopropyltrimethoxysilane, and NXT-Z100, NXT-Z45 and NXTmanufactured by Momentive Performance Materials; vinyl silane couplingagents such as vinyltriethoxysilane; amino silane coupling agents suchas 3-aminopropyltriethoxysilane; glycidoxy silane coupling agents suchas γ-glycidoxypropyltriethoxysilane; nitro silane coupling agents suchas 3-nitropropyltrimethoxysilane; and chloro silane coupling agents suchas 3-chloropropyltrimethoxysilane. These silane coupling agents may beused alone or may be used in combination with two or more thereof. Amongthem, sulfide silane coupling agents and mercapto silane coupling agentsare preferable from the viewpoint of their strong binding force withsilica and excellent low heat build-up characteristic.

When the rubber composition comprises the silane coupling agent, thecontent of the silane coupling agent is preferably not less than 2 partsby mass, more preferably not less than 3 parts by mass based on 100parts by mass of the silica. When the content of the silane couplingagent is less than 2 parts by mass, there is a tendency that an effectof improving dispersion of the silica is not obtained sufficiently. Onthe other hand, the content of the silane coupling agent is preferablynot more than 25 parts by mass, more preferably not more than 20 partsby mass. When the content of the silane coupling agent exceeds 25 partsby mass, an effect for a cost tends not to be obtained.

In addition to the above-mentioned components, the crosslinked rubbercomposition according to the present invention can comprise compoundingagents generally used in manufacturing crosslinked rubber composition,for example, a resin component, oils, zinc oxide, stearic acid,antioxidants, wax, sulfur donator, a vulcanizing agent, a vulcanizationaccelerator and the like.

The crosslinked rubber composition according to the present invention ischaracterized in that the volume of the low density region when extendedby an applied stress of 1.5 MPa is 35% or more, and that the volume ofthe void portion when extended by an applied stress of 3.0 MPa is 7.5%or less, and that the ratio of a component having a weight-averagemolecular weight of not less than 1,000,000 in a molecular weightdistribution measured by gel permeation chromatography is 40% by mass ormore. A method of measuring a molecular weight distribution by gelpermeation chromatography is not limited particularly, and aconventional measuring method can be employed.

In a crosslinked rubber composition having a large volume of the lowdensity region, a crosslinking structure thereof is highly uniform, andtherefore, a stress is not concentrated in a specific region and isdispersed. A crosslinked rubber composition having a small region of thegenerated void portion is excellent in durability against an externalstress such as breaking resistance and abrasion resistance. Further, ina crosslinked rubber composition having a high ratio of a componenthaving a weight-average molecular weight of not less than 1,000,000, afree motion of rubber molecules is restricted and breakage hardlyarises. The crosslinked rubber composition according to the presentinvention can realize high abrasion resistance since the volume of thelow density region when extended by an applied stress of 1.5 MPa islarge, the volume of the void portion when extended by an applied stressof 3.0 MPa is small, and a ratio of a component having a weight-averagemolecular weight of not less than 1,000,000 is high.

The volume of the low density region when extended by an applied stressof 1.5 MPa is preferably 40% or more. On the other hand, the volume ofthe low density region when extended by an applied stress of 1.5 MPa ispreferably 95% or less.

The volume of the void portion when extended by an applied stress of 3.0MPa is preferably 7.0% or less.

In order to realize the condition such that the volume of the lowdensity region when extended by an applied stress of 1.5 MPa is 35% ormore and the volume of the void portion when extended by an appliedstress of 3.0 MPa is 7.5% or less, it is necessary to make a crosslinkedstate in the rubber composition uniform. Example of a means for making acrosslinked state in the rubber composition uniform include a method ofincreasing a kneading time or increasing the number of kneading steps,thereby making a dispersed state of a vulcanizing agent and/or avulcanization accelerator uniform.

The low density region and the void portion will be explained below.First, if a stress applied to the crosslinked rubber composition exceedsan inherent critical value of the crosslinked rubber composition, adeviation in density in the crosslinked rubber composition arises,thereby generating a low density region inside thereof. There are areversible portion and an irreversible portion in this low densityregion.

The reversible portion is a low density region to be generated in thecase where an applied stress is small (1.5 MPa), and disappears by arelease of the stress and an original uniform density distribution isrecovered. Here, the low density region to be generated in the casewhere an applied stress is small is a region wherein a density is notless than 0.1 time an average density of the crosslinked rubbercomposition before extended and not more than 0.8 time an averagedensity of the crosslinked rubber composition before extended. Therubber test piece can be evaluated in good accuracy by evaluating thedistribution of this reversible portion.

The irreversible portion is a low density region to be generated in thecase where an applied stress is large (3.0 MPa), in which an innerstructure of the crosslinked rubber composition (bonding of molecularchains) is partially broken by the stress and even after the stress isreleased, the low density region is left without recovering an originalstate. Here, in the irreversible portion, the void portion is a regionwhere the inner structure was broken extremely and a density is not lessthan 0 time an average density of the crosslinked rubber compositionbefore extended and not more than 0.8 time an average density of thecrosslinked rubber composition before extended. The rubber test piececan be evaluated in good accuracy by evaluating the distribution of thisvoid portion.

A method of evaluating the volume of the low density region whenextended by an applied stress of 1.5 MPa and the volume of the voidportion when extended by an applied stress of 3.0 MPa is not limitedparticularly as far as a density distribution of the crosslinked rubbercomposition when extended can be evaluated, and an evaluation methodusing an X-ray computerized tomography is preferable.

The evaluation method of a density distribution of the crosslinkedrubber composition when extended using an X-ray computerized tomographywill be described by referring to the attached drawings. FIG. 1 is aschematic perspective view of an example of an evaluation device to beused for the evaluation method. The evaluation device 1 as shown in FIG.1 comprises a stress application means 2, a photographing means 3 and anevaluation means 4.

The stress application means 2 applies a stress for extending the rubbertest piece 10 to generate a low density region inside the rubber testpiece 10.

It is preferable that the stress application means 2 comprises a pair ofjigs 21, 22 for fixing the test piece 10 and a drive means 23 for movingthe jig 21 and the jig 22 relatively to each other, thereby applying astress to the test piece 10. With the one jig 21 being fixed, the drivemeans 23 allows another jig 22 to move in an axial direction of the testpiece 10. Thus, a stress for extending the test piece 10 in its axialdirection is applied.

The stress applied to the test piece 10 is detected with a load cell(not shown) or the like. A position and a type of the load cell areselected optionally. A predetermined stress is applied to the rubbertest piece 10 with the stress application means 2. The drive means 23 isconfigured to enable the rubber test piece 10 and the jigs 21, 22 to berotated relative to the axis of the rubber test piece 10.

The photographing means 3 irradiates an X-ray onto the test piece 10 totake a projection image. The photographing means 3 has an X-ray tube 31for irradiating an X-ray and a detector 32 for detecting the X-ray andconverting it into an electric signal. While the rubber test piece 10and the jigs 21, 22 being rotated relative to the axis of the rubbertest piece 10, the photographing means 3 photographs plural projectionimages and the projection images of the whole perimeter of the testpiece 10 can be obtained.

The detector 32 has a phosphor 32 a for converting an x-ray into avisible light. A decay time of the phosphor 32 a is preferably 100 ms orless. In the case where the decay time of the phosphor 32 a exceeds 100ms, when photographing plural projection images continuously whilerotating the test piece 10 and others relative to the axis of the testpiece 10, a residual image of the previously photographed projectionimage may have an adverse effect on a projection image to bephotographed thereafter. From this point of view, the decay time of thephosphor 32 a is more desirably 50 ms or less, further desirably 10 msor less.

The evaluation means 4 evaluates performance of the crosslinked rubbercomposition based on the density distribution determined from theprojection image. To the evaluation means 4, for example, a computer 40is applied. The computer 40 comprises a main body 41, a keyboard 42 anda display device 43. This main body 41 is provided with storage devicessuch as a central processing unit (CPU), an ROM, an operation memory anda hard disc. A process procedure (program) for performing a simulationmethod according to this embodiment has been stored in the storagedevices beforehand.

FIG. 2 is a flow chart showing a process procedure of an evaluationmethod of a density distribution of a crosslinked rubber composition atelongation thereof using the evaluation device 1. The evaluation methodof a density distribution includes steps S1 and S2 for applying a stressto the test piece 10 to generate a deviation of density (a low densityregion) inside the rubber test piece 10, photographing steps S3 and S4for irradiating the rubber test piece 10 with an X-ray and photographinga projection image, and evaluation steps S5 and S6 for evaluating adensity distribution of the crosslinked rubber composition based on adensity distribution determined from the projection images.

In the step S1, the rubber test piece 10 is fixed to the jigs 21, 22.

A shape of the rubber test piece 10 is not limited particularly, but ispreferably in a form of a column or a rectangular parallelepiped, morepreferably in a form of a column for the reason that the rubber testpiece 10 has symmetry and that measurement results being high inreproducibility can be easily obtained.

The rubber test piece 10 has a diameter which is preferably five times,more preferably ten times, further preferably twenty times the length inits axial direction. According to such test piece 10, a deformation of aside surface of the rubber test piece 10 is restricted when a stress isapplied to the rubber test piece 10. As a result, a volume of the rubbertest piece 10 increases and a very large stress is applied inside therubber test piece 10. Therefore, a low density region is prone to begenerated inside the test piece 10, and evaluation of performance of anelastic material can be performed rapidly and easily.

The rubber test piece 10 is fixed to the both jigs 21, 22 in a state ofbeing interposed therebetween. The top surface of the rubber test piece10 is fixed to the bottom surface of the jig 21, and the bottom surfaceof the test piece 10 is fixed to top surface of the jig 22. The fixingmethod can be selected adequately depending on a test environment andthe like. For example, fixing with an adhesive or fixing by adhesion dueto vulcanization of an elastic material constituting the test piece 10can be applied. Further, the test piece 10 may be fixed to the jigs 21,22 by providing corresponding engaging parts on the top surface and thebottom surface and the bottom surface and the top surface, respectivelyand connecting the engaging parts.

In the step S2, as shown in FIG. 1, the jig 21 and the jig 22 move in anaxial direction of the rubber test piece 10, namely the jig 22 moves ina direction apart from the jig 21, thereby extending the rubber testpiece 10. If the stress exceeds a critical value of the crosslinkedrubber composition, there arises a deviation of density in the rubbertest piece 10 and a low density region is generated inside the testpiece 10.

In the present invention, a stress for extending the test piece 10 inorder to determine a distribution of a low density region to begenerated with a small applied stress is 1.5 MPa. Further, a stress forextending the test piece 10 in order to determine a distribution of avoid portion which is, among low density regions to be generated with alarge applied stress, a region having an inside structure extremelydestroyed is 3.0 MPa.

In the step S3, X-rays are irradiated onto the rubber test piece 10 fromthe X-ray tube 31. The X-rays passes through the rubber test piece 10and are detected by the detector 32. The detector 32 converts thedetected X-rays into an electric signal and outputs the electric signalto the computer 40.

A luminance of the X-rays irradiated from the X-ray tube 31 onto therubber test piece 10 is greatly related to an S/N ratio of X-rayscattering data. In the case where the luminance of X-rays is small,there is a tendency that a signal strength is weaker than a statisticalerror of X-rays, and even if a measuring time is increased, it may bedifficult to obtain data showing a sufficiently good S/N ratio. Fromthis point of view, the luminance of X-rays is preferably 10¹⁰photons/s/mrad²/mm²/0.1% bw or more.

In the step S4, the electric signal outputted from the detector 32 isprocessed by the computer 40 to obtain a projection image.

In the step S5, the projection image is reconstructed by the computer 40to obtain a three-dimensional tomogram of the rubber test piece 10. Thenin the step S6, the density distribution of the crosslinked rubbercomposition when extended can be evaluated from the tomogram, andvolumes of the reversible portion and the void portion can be obtained.

As mentioned above, in the crosslinked rubber composition of the presentinvention, the ratio of a component having a weight-average molecularweight of not less than 1,000,000 in a molecular weight distributionmeasured by gel permeation chromatography is 40% by mass or more.

The above-mentioned ratio of a component having a weight-averagemolecular weight of not less than 1,000,000 is preferably 45% by mass ormore, more preferably 50% by mass or more, further preferably 60% bymass or more. An upper limit of the ratio of a component having aweight-average molecular weight of not less than 1,000,000 is notlimited particularly, and yet is preferably 90% by mass or less, morepreferably 80% by mass or less from the viewpoint of processability.

The crosslinked rubber composition of the present invention can beprepared by a usual method as far as the crosslinked rubber compositionexhibiting the above-mentioned volume of a low density region and volumeof a void portion when extended and the above-mentioned ratio of acomponent having a weight-average molecular weight of not less than1,000,000 can be obtained. The crosslinked rubber composition can beprepared, for example, by a method of kneading the above-mentionedcomponents other than the vulcanizing agent and the vulcanizationaccelerator with a generally well-known kneading machine such as aBanbury mixer, a kneader or an open roll and then adding the vulcanizingagent and the vulcanization accelerator followed by further kneading andthen conducting vulcanization, or by other method.

For the reason that a crosslinked state of the crosslinked rubbercomposition can be made uniform and a rubber composition having many lowdensity regions when extended and a small volume of a void portion whenextended is easily obtained, preferred is a preparation method includinga step of initiating kneading of a rubber component, a sulfur donatorand a sulfur atom-containing vulcanization accelerator and subsequentlyadding a filler to the obtained kneaded product and kneading at atemperature of 120° C. or higher.

The sulfur donator is, for example, an elementary sulfur or a sulfurcompound releasing an active sulfur under vulcanization conditions (forexample, 150° C., 1.5 MPa) or under a temperature and pressure lowerthan them. In other words, this sulfur compound is a compound generallyexhibiting a function as a vulcanizing agent under the vulcanizationconditions (for example, 150° C., 1.5 MPa) or under a temperature andpressure lower than them. It is noted that this released active sulfurforms a part of a pendant structure described later.

The above-mentioned sulfur atom-containing vulcanization acceleratorindicates a vulcanization accelerator including a sulfur atom bonded toother molecule by a single bond. Among sulfur atom-containingvulcanization accelerators, there are those releasing an active sulfurand those not releasing an active sulfur, and from the viewpoint ofinhibiting advancement of a crosslinking reaction during the kneading,sulfur atom-containing vulcanization accelerators not releasing anactive sulfur are preferable.

By initiating the kneading of the rubber component, the sulfur donatorand the sulfur atom-containing vulcanization accelerator before kneadingthe rubber component and the filler, adsorption of the sulfur donatorand the sulfur atom-containing vulcanization accelerator by the fillercan be prevented and therefore, the sulfur donator and the sulfuratom-containing vulcanization accelerator can be dispersed in the rubbercomponent efficiently. In the above-mentioned preparation method, thefiller is added to a kneaded product obtained by kneading the rubbercomponent, the sulfur donator and the sulfur atom-containingvulcanization accelerator, followed by kneading at a kneadingtemperature of 120° C. or higher. The active sulfur is released from thesulfur donator under the kneading temperature of 120° C. or higher and amechanical shearing force during the kneading. The sulfur donator, thesulfur atom-containing vulcanization accelerator and the rubbercomponent react with each other, which leads to a state of the whole ora part of the sulfur atom-containing vulcanization accelerator(hereinafter referred to as “vulcanization accelerator residue”) beingbonded to the rubber component, namely a state of a pendant structurebeing formed, in which “—S-vulcanization accelerator residue” is bondedto the rubber component. In this reaction mechanism, it is conjecturedthat the released active sulfur reacts with the sulfur atom of thesulfur atom-containing vulcanization accelerator to form a structurehaving two or more bonded sulfur atoms and the structure portion reactswith the double bond portion of the rubber component. By kneading in astate that the pendant structure is formed, the rubber component and thevulcanization accelerator residue move together, and therefore,uniformity of a dispersed state of the vulcanization accelerator residuecan be enhanced in the entire rubber composition. Thus, in theabove-mentioned preparation method, uniformity of a crosslinking densityat the time of vulcanization can be enhanced. It is noted that thekneading temperature as used herein is an actually measured temperatureof the rubber composition in the kneading machine, and a surfacetemperature of the rubber composition can be measured with a non-contacttype temperature sensor or the like.

The preparation method is featured, for example, by a point that thekneading of the rubber component, the sulfur donator and the sulfuratom-containing vulcanization accelerator is initiated before kneadingthe filler and a point that after adding the filler, kneading isperformed at the kneading temperature of 120° C. or higher. As far asthe above-mentioned requirements are satisfied, any materials may beadded in any step. For example, in the case where the kneading stepcomprises two steps including a step X and a step F, the kneading stepmay be such that kneading of the rubber component, the sulfur donatorand the sulfur atom-containing vulcanization accelerator is initiated atan initial stage of the step X, the filler is added in the midst of thestep X, and kneading is conducted at the kneading temperature of 120° C.or higher, and thereafter, the step F is performed. Further, forexample, in the case where the kneading step comprises three stepsincluding the step X, a step Y and the step F, the kneading step may besuch that kneading of the rubber component, the sulfur donator and thesulfur atom-containing vulcanization accelerator is initiated in thestep X, the filler is added in the following step Y and kneading isconducted at the kneading temperature of 120° C. or higher, andthereafter, the step F is performed. Further, example of other kneadingstep including three steps may be such that kneading of the rubbercomponent, the sulfur donator and the sulfur atom-containingvulcanization accelerator is initiated at an initial stage of the stepX, the filler is added in the midst of the step X and kneading isconducted at the kneading temperature of 120° C. or higher, andthereafter, the step Y and the step F are performed, or may be such thatkneading of the rubber component, the sulfur donator and the sulfuratom-containing vulcanization accelerator is initiated at an initialstage of the step X, the filler is added in the midst of the step X, thefiller is further added and kneading is conducted at the kneadingtemperature of 120° C. or higher in the following step Y, andthereafter, the step F is performed. It is noted that re-milling may beperformed between the respective steps.

The temperature of kneading the rubber component, the sulfur donator andthe sulfur atom-containing vulcanization accelerator is not limitedparticularly. The kneading temperature is preferably lower than 160° C.,more preferably 150° C. or lower from the viewpoint of inhibiting thecrosslinking reaction by the sulfur donator and the sulfuratom-containing vulcanization accelerator from proceeding.

Further, the kneading time of the rubber component, the sulfur donatorand the sulfur atom-containing vulcanization accelerator before addingthe filler to the rubber component is not limited particularly. Thekneading time is, for example, 10 seconds or more from the viewpoint ofenhancement of dispersibility.

The kneading temperature after addition of the filler is preferably 170°C. or lower from the viewpoint of inhibiting the crosslinking reactionfrom progressing excessively.

Further, the kneading time after addition of the filler to the rubbercomponent and reach of the kneading temperature to 120° C. is notlimited particularly, and from the viewpoint of enhancement ofdispersibility, is, for example, two minutes or more. Here, the kneadingtime is a time from a point of time when the kneading temperaturereached 120° C. after addition of the filler to the rubber component upto a point of time when all the steps of the kneading step has beencompleted. For example, when the kneading temperature reached 120° C.after addition of the filler to the rubber component in the step X, thekneading time is a time from this point of time up to a point of timewhen the step F is completed.

As mentioned above, an elementary sulfur and/or the above-mentionedsulfur compound releasing active sulfur can be used as the sulfurdonator. Examples of the elementary sulfur include powdered sulfur,precipitated sulfur, colloidal sulfur, surface-treated sulfur, insolublesulfur, and the like.

If too much amount of elementary sulfur is compounded as the sulfurdonator, the vulcanization reaction may progress excessively in thekneading step. Therefore, even when the elementary sulfur is used as thesulfur donator, a content thereof is preferably 0.1 part by mass or lessbased on 100 parts by mass of the rubber component. On the other hand,the content is preferably 0.05 part by mass or more from the viewpointof a breaking strength.

Examples of the sulfur compound functioning as the sulfur donatorinclude polymeric polysulfide represented by -(-M-S—C—)_(n)— and acompound which has a structure —S_(n)— (n≥2) having two or more ofsulfur atoms bonded by a single bond and releases an active sulfur.Examples of this compound include alkylphenol disulfide, morpholinedisulfide, thiuram vulcanization accelerators having —S_(n)— (n≥2)(tetramethylthiuram disulfide (TMTD), tetraethylthiuram disulfide(TETD), tetrabutylthiuram disulfide (TBTD), dipentamethylenethiuramdisulfide (DPTT), and the like), vulcanization accelerators such as2-(4′-morpholinodithio)benzothiazole (MDB) and polysulfide silanecoupling agent (for example, Si69(bis(3-triethoxysilylpropyl)tetrasulfide) manufactured by Degussa AG),and a sulfide compound represented by the following formula (1), (2) or(3).

wherein R¹s are the same or different, and represent monovalenthydrocarbon groups which have 3 to 15 carbon atoms and may have asubstituent, and n represents an integer of 2 to 6.

R¹s in the above formula (1) are monovalent hydrocarbon groups whichhave 3 to 15 carbon atoms and may have a substituent, and the number ofcarbon atoms is preferably from 5 to 12, more preferably from 6 to 10.The monovalent hydrocarbon groups R¹ may be either of a straight-chain,a branched chain or a cyclic one, and may be either of a saturated orunsaturated hydrocarbon group (an aliphatic, alicyclic or aromatichydrocarbon group or other group). Among these, aromatic hydrocarbongroups which may have a substituent are preferable.

Examples of preferred R¹s include an C₃₋₁₅ alkyl group, a substitutedC₃₋₁₅ alkyl group, a C₃₋₁₅ cycloalkyl group, a substituted C₃₋₁₅cycloalkyl group, an C₃₋₁₅ aralkyl group, a substituted C₃₋₁₅ aralkylgroup and the like, and among these, the aralkyl group and thesubstituted aralkyl group are preferable. Here, examples of the alkylgroup include a butyl group and an octyl group, an example of thecycloalkyl group includes a cyclohexyl group, and examples of thearalkyl group include a benzyl group and a phenethyl group. Examples ofa substituent include polar groups such as an oxo group (═O), a hydroxylgroup, a carboxyl group, a carbonyl group, an amino group, an acetylgroup, an amide group and an imide group.

Further, “n” in the formula (1) is an integer of from 2 to 6, and 2 or 3is preferable.

wherein R²s are the same or different, and represent divalenthydrocarbon groups which have 3 to 15 carbon atoms and may have asubstituent, and “m” represents an integer of 2 to 6.

In the above formula (2), R²s are divalent hydrocarbon groups which have3 to 15 carbon atoms and may have a substituent, and the number ofcarbon atoms is preferably from 3 to 10, more preferably from 4 to 8.The divalent hydrocarbon group R² may be either of a straight-chain, abranched chain or a cyclic one, and may be either of a saturated orunsaturated hydrocarbon group (an aliphatic, alicyclic or aromatichydrocarbon group or other group). Among these, aliphatic hydrocarbongroups which may have a substituent are preferable, and straight-chainaliphatic hydrocarbon groups are more preferable.

Examples of the R²s include an C₃₋₁₅ alkylene group, a substituted C₃₋₁₅alkylene group and the like. Here, examples of alkylene groups include abutylene group, a pentylene group, a hexylene group, a heptylene group,an octylene group, a nonylene group and the like, and examples of thesubstituent include the same substituents as in R¹.

Further, “m” in the formula (2) is an integer of from 2 to 6, and 2 or 3is preferable.

Examples of the sulfide compounds represented by the above-mentionedformula (1) or (2) include N,N′-di(γ-butyrolactam)disulfide,N,N′-di(5-methyl-γ-butyrolactam)disulfide,N,N′-di(5-ethyl-γ-butyrolactam)disulfide,N,N′-di(5-isopropyl-γ-butyrolactam)disulfide,N,N′-di(5-methoxyl-γ-butyrolactam)disulfide,N,N′-di(5-ethoxyl-γ-butyrolactam)disulfide,N,N′-di(5-chloro-γ-butyrolactam)disulfide,N,N′-di(5-nitro-γ-butyrolactam)disulfide,N,N′-di(5-amino-γ-butyrolactam)disulfide,N,N′-di(δ-balerolactam)disulfide, N,N′-di(δ-caprolactam)disulfide,N,N′-di(ε-caprolactam)disulfide,N,N′-di(3-methyl-δ-caprolactam)disulfide,N,N′-di(3-ethyl-ε-caprolactam)disulfide,N,N′-di(3-isopropyl-ε-caprolactam)disulfide,N,N′-di(δ-methoxy-ε-caprolactam)disulfide,N,N′-di(3-ethoxy-ε-caprolactam)disulfide,N,N′-di(3-chloro-ε-caprolactam)disulfide,N,N′-di(δ-nitro-ε-caprolactam)disulfide,N,N′-di(3-amino-ε-caprolactam)disulfide,N,N′-di(ω-heptalactam)disulfide, N,N′-di(ω-octalactam)disulfide,dithiodicaprolactam, morpholine disulfide,N-benzyl-N-[(dibenzylamino)disulfanyl]phenylmethanamine(N,N′-dithiobis(dibenzylamine)), and the like. These sulfide compoundsmay be used alone or may be used in combination of two or more thereof.R³

S

_(k)R³  (3)wherein R³s are the same or different, and represent an alkyl group, abenzothiazolyl group, an amino group, a morpholino group, adialkylthiocarbamoyl group or a group represented by the followingformula (4). “k” represents an integer of 2 to 6.

wherein R⁴s are the same or different, and represent an alkyl group, abenzothiazolylsulfide group, a cycloalkyl group or a hydrogen atom.

R³s in the above formula (3) are the same or different, and represent analkyl group, a benzothiazolyl group, an amino group, a morpholino group,a dialkylthiocarbamoyl group or a group represented by the above formula(4). Among these, preferable are an alkyl group having 1 to 10 carbonatoms, a benzothiazolyl group, an amino group, a morpholino group or adialkylthiocarbamoyl group (The alkyl groups are the same or different,and are alkyl groups having 1 to 10 carbon atoms.).

Examples of the alkyl group having 1 to 10 carbon atoms and alkyl groupshaving 1 to 10 carbon atoms in the dialkylthiocarbamoyl group includemethyl, ethyl, n-propyl, isopropyl, n-butyl, iso-butyl, sec-butyl,tert-butyl, pentyl, hexyl, heptyl, 2-ethylhexyl, octyl, nonyl and thelike.

Examples of more preferable R³s in the above formula (3), which are thesame or different, include a benzothiazolyl group, a morpholino groupand a dialkylthiocarbamoyl group (The alkyl groups are the same ordifferent, and are alkyl groups having 1 to 5 carbon atoms.). Examplesof further preferable R³s in the above formula (3), which are the sameor different, include a benzothiazolyl group or a dialkylthiocarbamoylgroup (The alkyl groups are the same or different, and are alkyl groupshaving 1 to 5 carbon atoms.).

“k” in the formula (3) is an integer of from 2 to 6, and 2 or 3 isfurther preferable.

R⁴s in the above formula (4) are the same or different, and each of themis an alkyl group, a benzothiazolylsulfide group, a cycloalkyl group ora hydrogen atom. The alkyl group is preferably an alkyl group having 1to 10 carbon atoms, and the cycloalkyl group is preferably a cycloalkylgroup having 1 to 5 carbon atoms.

Examples of the sulfide compound represented by the formula (3) includetetramethylthiuram disulfide, tetramethylthiuram disulfide,tetrabutylthiuram disulfide, 2-(morpholinodithio)benzothiazol,dibenzothiazolyl disulfide, N-cycohexyl-2-benzothiazolylsulfeneamide andthe like, and dibenzodithiazolyl disulfide can be used suitably. Thesemay be used alone or may be used in combination with two or morethereof.

When using the sulfur compound functioning as the sulfur donator, acontent thereof is preferably not less than 0.1 part by mass, morepreferably not less than 0.2 part by mass based on 100 parts by mass ofthe rubber component for the reason that formation of a pendant typestructure is accelerated. On the other hand, the content is preferablynot more than 5 parts by mass, more preferably not more than 3 parts bymass, further preferably not more than 2 parts by mass from theviewpoint of inhibiting gelation during the kneading.

The vulcanization accelerator functioning as the sulfur donator includevulcanization accelerators having sulfur atom bonded to other moleculeby a single bond. Therefore, the sulfur atom-containing vulcanizationaccelerators functioning as the sulfur donator have both functions ofthe sulfur donator and the sulfur atom-containing vulcanizationaccelerators, and also by compounding much amount of a single sulfuratom-containing vulcanization accelerator functioning as the sulfurdonator or by combination use of two or more of such vulcanizationaccelerators, formation of a pendant type structure is possible.However, when compounding much amount of the sulfur atom-containingvulcanization accelerator functioning as the sulfur donator, acrosslinking reaction may progress excessively during the kneading, andwhen compounding a small amount of the sulfur atom-containingvulcanization accelerator functioning as the sulfur donator, an effectof making a crosslinking density uniform may be hard to obtain.Therefore, it is preferable that the sulfur donator and the sulfuratom-containing vulcanization accelerator which are kneaded beforeaddition of the filler are a sulfur donator (a sulfur atom-containingvulcanization accelerator functioning as the sulfur donator and/or othersulfur donator) and a sulfur atom-containing vulcanization acceleratorreleasing no sulfur.

The sulfur atom-containing vulcanization accelerator releasing no sulfuris a sulfur atom-containing vulcanization accelerator which does notrelease sulfur under vulcanization conditions (for example, 150° C., 1.5MPa) or under a temperature and pressure lower than the vulcanizationconditions. In other words, this sulfur atom-containing vulcanizationaccelerator releasing no sulfur is a sulfur atom-containingvulcanization accelerator not exhibiting a function as a vulcanizingagent under vulcanization conditions (for example, 150° C., 1.5 MPa) orunder a temperature and pressure lower than the vulcanizationconditions.

Examples of the sulfur atom-containing vulcanization acceleratorreleasing no sulfur include thiazole-based vulcanization accelerators(2-mercaptobenzothiazole (MBT), zinc salt of 2-mercaptobenzothiazole(ZnMBT), cyclohexylamine salt of 2-mercaptobenzothiazole (CMBT) and thelike); sulfenamide-based vulcanization accelerators(N-cyclohexyl-2-benzothiazolylsulfenamide (CBS),N-(tert-butyl)-2-benzothiazolyl sulfenamide (TBBS),N,N-dicyclohexyl-2-benzothiazolyl sulfenamide, and the like);vulcanization accelerators of tetramethylthiuram monosulfide (TMTM);dithiocarbamate-based vulcanization accelerators (piperidiniumpentamethylene dithiocarbamate (PPDC), zinc dimethyldithiocrbamate(ZnMDC), zinc diethyldithiocarbamate (ZnEDC), zincdibutyldithiocarbamate (ZnBDC), zinc N-ethyl-N-phenyldithiocarbamate(ZnEPDC), zinc N-pentamethylenedithiocarbamate (ZnPDC), sodiumdibutyldithiocarbamate (NaBDC), copper dimethyldithiocarbamate (CuMDC),iron dimethyldithiocarbamate (FeMDC), tellurium diethyldithiocarbamate(TeEDC) and the like); and the like. It is noted thatdi-2-benzothiazolyl sulfenamide (MBTS) being a thiazole-basedvulcanization accelerator has —S_(n)— (n≥2) and is a sulfur-releasingvulcanization accelerator, but does not exhibit a function as avulcanizing agent against a natural rubber and a butadiene rubber in thecase of a usual compounding amount, and therefore, can be usedequivalently to a sulfur atom-containing vulcanization acceleratorreleasing no sulfur.

A content of the sulfur atom-containing vulcanization accelerator ispreferably not less than 1.0 part by mass, more preferably not less than1.5 parts by mass based on 100 parts by mass of the rubber component forthe reason that a vulcanization reaction progresses efficiently in thevulcanization step. On the other hand, the content is preferably notmore than 5 parts by mass, more preferably not more than 3 parts by massfrom the viewpoint of inhibiting scorching and precipitation on asurface of the rubber.

In the above-mentioned preparation method, it is preferable that afteradding the filler to the rubber component and kneading at a kneadingtemperature of 120° C. or higher, an additional sulfur donator isfurther kneaded. By adding the additional sulfur donator, whileinhibiting the crosslinking reaction from progressing excessively duringthe kneading, the crosslinking reaction can be progressed sufficientlyduring the vulcanization.

The additional sulfur donator is added in the following step F afteradding the filler to the rubber component and kneading at a kneadingtemperature of 120° C. or higher. The additional sulfur donator may bethe same kind of sulfur donator as one kneaded before adding the fillerto the rubber component or may be a separate kind of sulfur donator.Examples thereof include elementary sulfur such as powdered sulfur,precipitated sulfur, colloidal sulfur, surface-treated sulfur, insolublesulfur, or the like.

A content of the additional sulfur donator is not limited particularly,and is preferably not less than 0.5 part by mass, more preferably notless than 0.8 part by mass based on 100 parts by mass of the rubbercomponent for the reason that a vulcanization reaction progressesefficiently in the vulcanization step. On the other hand, the content ispreferably not more than 3.0 parts by mass, more preferably not morethan 2.5 parts by mass for the reason that abrasion resistance isexcellent.

When adding the additional sulfur donator in the step F, a usualvulcanizing accelerator may be added. Examples of the usual vulcanizingaccelerator include thiuram-based disulfides and polysulfides beingsulfur atom-containing vulcanization accelerators; guanidine-,aldehyde-amine-, aldehyde-ammonia- and imidazoline-based vulcanizationaccelerators being vulcanization accelerators having no sulfur atom; andthe like.

A content of the vulcanization accelerator to be added in the step F ispreferably not less than 0.1 part by mass based on 100 parts by mass ofthe rubber component. A mass ratio of a compounding amount of thevulcanization accelerator to be added in the step F to a compoundingamount of the sulfur atom-containing vulcanization accelerator to bekneaded before adding the filler to the rubber component is preferablyhigher than 0% and not more than 80%, further preferably not more than60%. When the mass ratio is not more than 80%, a crosslinked rubbercomposition inhibiting scorching and having excellent breakingresistance and abrasion resistance can be obtained.

The crosslinked rubber composition of the present invention can be usednot only for tire members such as a tread, an under tread, a carcass, aside wall and a bead of a tire but also a vibration-proof rubber, abelt, a hose and other rubber products in a rubber industry. Inparticular, a tire having a tread composed of the crosslinked rubbercomposition of the present invention is preferable since it hasexcellent abrasion resistance.

A tire produced using the crosslinked rubber composition of the presentinvention can be produced by a usual method using an un-crosslinkedrubber composition. Namely, the tire can be produced by subjecting anun-crosslinked rubber composition prepared by compounding theabove-mentioned additives with the diene rubber component according tonecessity, to extrusion processing to a shape of a tread or the like,and then laminating together with other tire members on a tire buildingmachine and forming by a usual forming method, thus forming anunvulcanized tire, and heating and compressing this unvulcanized tire ina vulcanizer.

A structure of the pneumatic tire of the present invention is notlimited particularly, and can be a structure of a conventional pneumatictire. Namely, the pneumatic tire of the present invention can be made byusing a member produced from the crosslinked rubber composition of thepresent invention on at least one of tire members constituting apneumatic tire having a conventional structure. In particular, it ispreferable that the pneumatic tire comprises bead cores provided on apair of right and left bead portions, respectively; a carcass plyextending from a crown portion to the both bead portions through bothside wall portions and moored to the bead cores; an inner liner disposedat an inner side than the carcass ply in a direction of a tire diameter;and a tread disposed at an outer side than the carcass ply in adirection of a tire diameter and having a volume of the low densityregion of 35% or more at elongation by an applied stress of 1.5 MPa, avolume of the void portion of 7.5% or less at elongation by an appliedstress of 3.0 MPa and a ratio of 40% by mass or more of a componenthaving a weight-average molecular weight of not less than 1,000,000 in amolecular weight distribution measured by gel permeation chromatography.It is noted that the above-mentioned tread is a portion coming intocontact with a road surface and in the case where the tread is composedof two or more different crosslinked rubber compositions and at leastone of the crosslinked rubber compositions is the crosslinked rubbercomposition of the present invention, an obtained tire is the tire ofthe present invention.

EXAMPLE

The present invention will be described based on Examples, but thepresent invention is not limited thereto only.

A variety of chemicals used in Examples and Comparative Examples will beexplained below.

SBR1: prepared in accordance with a method of preparing a modified SBR1mentioned later (S-SBR, styrene content: 26% by mass, vinyl content:59%, Tg: −25° C., Mw: 4×10⁵)

SBR2: SLR6430 (S-SBR, styrene content: 40% by mass, vinyl content: lessthan 25%, Tg: −40° C., Mw: 12×10⁵) manufactured by Dow CorningCorporation

BR: BR150B manufactured by Ube Industries, Ltd.

Silica: ULTRASIL VN3 (N₂SA: 175 m²/g) manufactured by Evonik Degussa

Silane coupling agent: Si266 (bis(3-triethoxysilylpropyl)disulfide)manufactured by Evonik Degussa

Carbon black: DIABLACK I (N₂SA: 98 m²/g, DBP oil absorption: 124 ml/100g) manufactured by Mitsubishi Chemical Corporation

Zinc oxide: Zinc Oxide No. 2 manufactured by Mitsui Mining & SmeltingCo., Ltd.

Stearic acid: Stearic acid beads “Tsubaki” manufactured by NOFCorporation

Antioxidant: OZONONE 6C(N-(1,3-dimethylbutyl)-N-phenyl-p-phenylenediamine, 6PPD) manufacturedby Seiko Chemical Co., Ltd.

Oil: Diana Process Oil AH-24 manufactured by Idemitsu Kosan Co., ltd.

Elementary sulfur: Powder sulfur manufactured by Tsurumi ChemicalIndustry Co., Ltd.

Vulcanization accelerator: Nocceler NS (TBBSN-tert-butyl-2-benzothiazolylsulfeneamide) manufactured by OUCHI SHINKOCHEMICAL INDUSTRIAL CO., LTD.

Sulfur donator: Rhenogran CLD80 (caprolactam disulfide) manufactured byRhein Chemie Corporation

A variety of chemicals used in the preparation method of SBR1 will beexplained below.

Cyclohexane: Cyclohexane manufactured by Kanto Chemical Industry Co.,Ltd.

Pyrrolidine: Pyrrolidine manufactured by Kanto Chemical Industry Co.,Ltd.

Divinylbenzene: Divinylbenzene manufactured by SIGMA-ALDRICH JAPAN

1.6M n-butyllithium hexane solution: 1.6M n-butyllithium hexane solutionmanufactured by Kanto Chemical Industry Co., Ltd.

Isopropanol: Isopropanol manufactured by Kanto Chemical Industry Co.,Ltd.

Styrene: Styrene manufactured by Kanto Chemical Industry Co., Ltd.

Butadiene: 1,3-Butadiene manufactured by TAKACHIHO CHEMICAL INDUSTRIALCO., LTD.

Tetramethylethylenediamine: N,N,N′,N′-Tetramethylethylenediaminemanufactured by Kanto Chemical Industry Co., Ltd.

Modifying agent: 3-(N,N-dimethylaminopropyl)trimethoxysilane manufactureby Azmax Co.

Preparation Method of SBR1

Into a 100 ml vessel having been subjected to replacement with nitrogensufficiently were poured 50 ml of cyclohexane, 4.1 ml of pyrrolidine and8.9 ml of divinylbenzene, and 0.7 ml of 1.6M n-butyllithium hexanesolution was added to the mixture, followed by stirring at 0° C. Onehour after, isopropanol was added to terminate the reaction, followed byextraction and refining to obtain Monomer-A. Subsequently into a 1,000ml pressure resistant vessel having been subjected to replacement withnitrogen sufficiently were added 600 ml of cyclohexane, 12.6 ml ofstyrene, 71.0 ml of butadiene, 0.06 g of Monomer-A and 0.11 ml oftetramethylethylenediamine, and 0.2 ml of 1.6M n-butyllithium hexanesolution was added to the mixture, followed by stirring at 40° C. Threehours after, 0.5 ml of a modifying agent was added, and the mixture wasstirred. One hour after, 3 ml of isopropanol was added to terminatepolymerization. After adding 1 g of 2,6-tert-butyl-p-cresol into areaction solution, the solution was subjected to reprecipitaiontreatment with methanol, followed by drying by heating to obtain SBR1.

Examples 1 to 13 and Comparative Examples 1 to 3

According to compounding formulations shown in Tables 1 and 2, variouschemicals shown in the step X were kneaded with a 1.7 L Banbury mixer ata discharge temperature of 100° C. for 5.0 minutes (step X).Subsequently the kneaded product obtained in the step X and variouschemicals shown in the step Y were kneaded with a 1.7 L Banbury mixer atnot less than 140° C. for 30 seconds and further kneaded at a dischargetemperature of 150° C. for 3 minutes (step Y). Then, the kneaded productobtained in the step Y and various chemicals shown in the step F werekneaded with an open roll at about 80° C. for 3 minutes (step F) toobtain an unvulcanized rubber composition. The obtained unvulcanizedrubber composition was extrusion-molded into a form of a tread using anextruder with an extrusion nozzle having a specific shape, and anextrudate was laminated with other tire members to form an unvulcanizedtire, followed by press-vulcanization at 170° C. for 12 minutes toproduce a test tire (tire size: 195/65R15). The following evaluationswere made using the obtained test tires. The results of the evaluationsare shown in Tables 1 and 2.

Volume of a Low Density Region

Rubber test pieces in a form of column having a diameter of 10 mm and aheight of 1 mm were prepared by cutting tread portions of test tires,and each rubber test piece was fixed to the jigs shown in FIG. 1 andextension of the test piece was initiated. At the time when extendedwith an applied stress of 1.5 MPa, X-rays (luminance: 10¹⁶photons/s/mrad²/mm²/0.1% bw) were irradiated and computerized tomographywas conducted. A volume ratio of the low density region in the extendedrubber test piece was calculated from a density distribution obtainedfrom a three-dimensional tomogram prepared by reconstructing thephotographed images. It is noted that the X-ray computerized tomographywas performed in a large synchrotron radiation facility SPring-8Beamline BL20B2, and P43(Gd₂O₂S: Tb), in which a decay time of aphosphor was 1 ms, was used as a phosphor. CT reconstruction wasperformed by a Convention Back Projection method by laminating 200tomograms having a thickness of 10 μm. The low density region is aregion having a density of 0.1 to 0.8 assuming that an average densityof the rubber test piece before extended is 1.

Volume of Void Region

Rubber test pieces in a form of column having a diameter of 10 mm and aheight of 1 mm were prepared by cutting tread portions of test tires,and each rubber test piece was fixed to the jigs shown in FIG. 1 andextension of the test piece was initiated. At the time when extendedwith an applied stress of 3.0 MPa, X-rays (luminance: 10¹⁶photons/s/mrad²/mm²/0.1% bw) were irradiated and computerized tomographywas conducted. A volume ratio of the void portion in the extended rubbertest piece was calculated from a density distribution obtained from athree-dimensional tomogram prepared by reconstructing the photographedimages. It is noted that the X-ray computerized tomography was performedin a large synchrotron radiation facility SPring-8 Beamline BL20B2, andP43(Gd₂O₂S: Tb), in which a decay time of a phosphor was 1 ms, was usedas a phosphor. CT reconstruction was performed by a Convention BackProjection method by laminating 200 tomograms having a thickness of 10μm. The void portion is a region having a density of 0 to 0.1 assumingthat an average density of the rubber test piece before extended is 1.

Molecular Weight

A weight-average molecular weight and a molecular weight distribution ofeach of rubber test pieces prepared by cutting tread portions of testtires were measured with a gel permeation chromatograph (GPC) (GPC-8000series manufactured by Tosoh Corporation; detector: differentialrefractometer; column: TSKGEL SUPERMALTPORE HZ-M manufactured by TosohCorporation, calibrated with standard polystyrene). A peak value of thehighest weight-average molecular weight measured and a ratio of acomponent having a weight-average molecular weight of not less than1,000,000 are shown.

Abrasion Resistance

Each of the test tires produced in the same manner as above was loadedon four wheels of a domestic FF vehicle and after in-vehicle running of8,000 km, a depth of the groove of each tire tread portion was measured.A traveling distance in which a groove depth of a tire was decreased by1 mm was calculated from an arithmetic mean of the groove depths of fourwheels. Assuming that an abrasion resistance index of ComparativeExample 1 is 100, a result of each compounding formulation is indicatedby an index with the following equation (abrasion resistance index). Thelarger the abrasion resistance index is, the more excellent the abrasionresistance is.Abrasion resistance index=(Traveling distance when a tire groove depthis decreased by 1 mm)/(Traveling distance of Comparative Example 1 whena tire groove depth is decreased by 1 mm)×100

TABLE 1 Compounding amount Ex. Com. Ex. (part by mass) 1 1 2 3 Step XSBR1 20 80 20 60 SBR2 60 — 60 20 BR 20 20 20 20 Vulcanizationaccelerator 1 — — 1 Sulfur donator 1.5 — — 1.5 Step Y Silica 70 70 70 70Silane coupling agent 6 6 6 6 Carbon black 5 5 5 5 Oil 15 15 15 15 Zincoxide 5 5 5 5 Stearic acid 3 3 3 3 Antioxidant 2 2 2 2 Vulcanizationaccelerator 1 1 1 1 Sulfur donator 1.5 1.5 1.5 1.5 Step F Sulfur 1.5 1.51.5 1.5 Vulcanization accelerator — 1 1 — Sulfur donator — 1.5 1.5 —Evaluation Volume of low density region (%) 75 24 30 75 Volume of voidportion (%) 5.8 9.1 7.9 5.8 Peak value of weight-average 150 40 150 150molecular weight (×10⁴) Ratio of component having a weight-averagemolecular weight of 60 0 60 20 1,000,000 or more (% by mass) Abrasionresistance index 161 100 121 130

TABLE 2 Compounding amount Examples (part by mass) 2 3 4 5 6 7 8 9 10 1112 13 Step X SBR1 22 24 26 28 22 24 26 28 20 20 20 20 SBR2 58 56 54 5260 60 60 60 60 60 60 60 BR 20 20 20 20 18 16 14 12 20 20 20 20Vulcanization accelerator 1 1 1 1 1 1 1 1 1 1 1 1 Sulfur donator 1.5 1.51.5 1.5 1.5 1.5 1.5 1.5 1.2 1.0 0.7 0.5 Step Y Silica 70 70 70 70 70 7070 70 70 70 70 70 Silane coupling agent 6 6 6 6 6 6 6 6 6 6 6 6 Carbonblack 5 5 5 5 5 5 5 5 5 5 5 5 Oil 15 15 15 15 15 15 15 15 15 15 15 15Zinc oxide 5 5 5 5 5 5 5 5 5 5 5 5 Stearic acid 3 3 3 3 3 3 3 3 3 3 3 3Antioxidant 2 2 2 2 2 2 2 2 2 2 2 2 Vulcanization accelerator 1 1 1 1 11 1 1 1 1 1 1 Sulfur donator 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.2 1.0 0.70.5 Step F Sulfur 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5Vulcanization accelerator — — — — — — — — — — — — Sulfur donator — — — —— — — — — — — — Evaluation Volume of low density region (%) 70 60 50 4070 60 50 40 75 75 75 75 Volume of void portion (%) 6.3 6.7 7.0 7.3 5.85.8 5.8 5.8 6.3 6.7 7.0 7.3 Peak value of weight-average 150 150 150 150150 150 150 150 150 150 150 150 molecular weight (×10⁴) Ratio ofcomponent having a weight-average molecular 58 56 54 52 60 60 60 60 6060 60 60 weight of 1,000,000 or more (% by mass) Abrasion resistanceindex 152 144 141 131 158 153 150 148 127 124 121 118

Examples 14 to 16

According to compounding formulations, and discharge temperatures andtimes shown in Table 3, chemicals shown in the step X were kneaded witha 1.7 L Banbury mixer (step X). Subsequently the kneaded productobtained in the step X and chemicals shown in the step Y were kneadedwith a 1.7 L Banbury mixer at discharge temperatures and times shown inTable 3 and further kneaded at a discharge temperature of 150° C. for 3minutes (step Y). Then, the kneaded product obtained in the step Y andchemicals shown in the step F were kneaded with an open roll atdischarge temperatures and times shown in Table 3 (step F) to obtain anunvulcanized rubber composition. The obtained unvulcanized rubbercomposition was extrusion-molded into a form of a tread using anextruder with an extrusion nozzle having a specific shape, and anextrudate was laminated with other tire members to form an unvulcanizedtire, followed by press-vulcanization at 170° C. for 12 minutes toproduce a test tire (tire size: 195/65R15). The following evaluationswere made using the obtained test tires. The results of the evaluationsare shown in Table 3.

TABLE 3 Compounding amount Examples (part by mass) 14 15 16 Step X SBR120 20 20 SBR2 60 60 60 BR 20 20 20 Kneading temperature (° C.) 100 100100 Kneading time (min) 5 5 5 Step Y Silica 70 70 70 Silane couplingagent 6 6 6 Carbon black 5 5 5 Oil 15 15 15 Zinc oxide 5 5 5 Stearicacid 3 3 3 Antioxidant 2 2 2 Vulcanization accelerator 1 1 1 Sulfurdonator 1.5 1.5 1.5 Kneading temperature (° C.) 160 160 160 Kneadingtime (min) 5 7 10 Step F Sulfur 1.5 1.5 1.5 Vulcanization accelerator 11 1 Sulfur donator 1.5 1.5 1.5 Kneading temperature (° C.) 80 80 80Kneading time (min) 5 7 10 Evaluation Volume of low density region (%)60 64 67 Volume of void portion (%) 7.3 7.1 6.9 Peak value ofweight-average 150 150 150 molecular weight (×10⁴) Ratio of componenthaving a weight-average molecular 60 60 60 weight of 1,000,000 or more(% by mass) Abrasion resistance index 110 113 116

From the results shown in Tables 1 to 3, it is seen that the pneumatictire of the present invention including a tread having a large volume ofthe low density region at elongation by an applied stress of 1.5 MPa, asmall volume of the void portion at elongation by an applied stress of3.0 MPa and a high ratio of a component having a weight-averagemolecular weight of not less than 1,000,000 in a molecular weightdistribution measured by gel permeation chromatography (GPC), and thecrosslinked rubber composition of the present invention having a largevolume of the low density region at elongation by an applied stress of1.5 MPa, a small volume of the void portion at elongation by an appliedstress of 3.0 MPa and a high ratio of a component having aweight-average molecular weight of not less than 1,000,000 in amolecular weight distribution measured by gel permeation chromatography(GPC), are the pneumatic tire and the crosslinked rubber composition,respectively being excellent in abrasion resistance.

EXPLANATION OF SYMBOLS

-   1 Evaluation device-   2 Stress application means-   3 Photographing means-   4 Evaluation means-   10 Rubber test piece

The invention claimed is:
 1. A pneumatic tire comprising: bead coresprovided on a pair of right and left bead portions, respectively; acarcass ply extending from a crown portion to the both bead portionsthrough both side wall portions and moored to the bead cores; an innerliner disposed at an inner side than the carcass ply in a direction of atire diameter; and a tread disposed at an outer side than the carcassply in a direction of a tire diameter and having a volume of the lowdensity region of 35% or more at elongation by an applied stress of 1.5MPa, a volume of the void portion of 7.5% or less at elongation by anapplied stress of 3.0 MPa and a ratio of 40% by mass or more of acomponent having a weight-average molecular weight of not less than1,000,000 in a molecular weight distribution measured by gel permeationchromatography, wherein the low density region is a region having adensity of 0.1 to 0.8 time the density of a crosslinked rubbercomposition before the elongation, and wherein the void portion is aregion having a density of 0 to 0.1 time the density of a crosslinkedrubber composition before the elongation, wherein the crosslinked rubbercomposition comprises a rubber component comprising two kinds ofstyrene-butadiene rubber, and butadiene rubber.
 2. The pneumatic tire ofclaim 1, wherein the crosslinked rubber composition further comprisessilica.
 3. The pneumatic tire of claim 1, wherein the crosslinked rubbercomposition further comprises solution-polymerized styrene-butadienerubber, wherein a content of the solution-polymerized styrene-butadienerubber is 80 parts by mass to 88 parts by mass.
 4. The pneumatic tire ofclaim 1, wherein a styrene content of the styrene-butadiene rubber is16% to 60% by mass and the vinyl content of the styrene-butadiene rubberis 10% to 90%.
 5. The pneumatic tire of claim 1, wherein the crosslinkedrubber composition further comprises an oil.
 6. The pneumatic tire ofclaim 1, wherein the crosslinked rubber composition further comprises asilane coupling agent.
 7. The pneumatic tire of claim 6, wherein thecrosslinked rubber composition further comprises silica, and wherein acontent of the silane coupling agent is 2 to 25 parts by mass based on100 parts by mass of silica.
 8. The pneumatic tire of claim 1, whereinthe Tg of the styrene-butadiene rubber is from −45° C. to 10° C.
 9. Thepneumatic tire of claim 1, wherein the weight-average molecular weightof the styrene-butadiene rubber is from 400,000 to 2,000,000.
 10. Thepneumatic tire of claim 1, wherein the crosslinked rubber compositionfurther comprises carbon black.
 11. The pneumatic tire of claim 10,wherein a content of the carbon black is 3 to 200 phr based on 100 partsby mass of the rubber component.
 12. The pneumatic tire of claim 1,wherein a content of the styrene-butadiene rubber in the rubbercomponent is 30% by mass to 90% by mass.
 13. The pneumatic tire of claim1, wherein a content of the butadiene rubber in the rubber component is10% by mass to 70% by mass.
 14. The pneumatic tire of claim 1, whereinthe crosslinked rubber composition further comprises: styrene-butadienerubber in the rubber component having a content of 30% by mass to 90% bymass; butadiene rubber in the rubber component having a content of 10%by mass to 70% by mass, wherein a styrene content of thestyrene-butadiene rubber in the rubber component is 16% to 60% by massand a vinyl content of the styrene-butadiene rubber is 10% to 90%;carbon black having a content of 3 to 200 phr based on 100 parts by massof the rubber component; silica having a content of 5 to 200 phr; silanecoupling agent.
 15. The pneumatic tire of claim 14, wherein a content ofsilane coupling agent is 2 to 25 parts by mass based on 100 parts bymass of the silica.
 16. The pneumatic tire of claim 15, wherein therubber component, a first portion of a sulfur donator and a firstportion of a vulcanization accelerator are kneaded in a first step toform a first product; wherein the first product and at least the silica,silane coupling agent, a second portion of the sulfur donator and asecond portion of the vulcanization accelerator are kneaded in a secondstep to form a second product; and wherein the second product and sulfurare added in a third step to form the crosslinked rubber composition.17. A crosslinked rubber composition having a volume of the low densityregion of 35% or more at elongation by an applied stress of 1.5 MPa, avolume of the void portion of 7.5% or less at elongation by an appliedstress of 3.0 MPa and a ratio of 40% by mass or more of a componenthaving a weight-average molecular weight of not less than 1,000,000 in amolecular weight distribution measured by gel permeation chromatography,wherein the crosslinked rubber composition comprises a rubber componentcomprising two kinds of styrene-butadiene rubber, and butadiene rubber,wherein the low density region is a region having a density of 0.1 to0.8 time the density of a crosslinked rubber composition before theelongation, and wherein the void portion is a region having a density of0 to 0.1 time the density of a crosslinked rubber composition before theelongation.
 18. The crosslinked rubber composition of claim 17, whereinthe volume of the low density region at elongation by an applied stressof 1.5 MPa is 40% or more, and wherein the volume of the void portion atelongation by an applied stress of 3.0 MPa is 7.3% or less.
 19. Thecrosslinked rubber composition of claim 17, wherein the volume of thelow density region at elongation by an applied stress of 1.5 MPa is 50%or more, and wherein the volume of the void portion at elongation by anapplied stress of 3.0 MPa is 7.0% or less.
 20. The crosslinked rubbercomposition of claim 17, wherein the volume of the low density region atelongation by an applied stress of 1.5 MPa is 60% or more, and whereinthe volume of the void portion at elongation by an applied stress of 3.0MPa is 6.3% or less.
 21. The crosslinked rubber composition of claim 17,wherein the volume of the low density region at elongation by an appliedstress of 1.5 MPa is 70% or more, and wherein the volume of the voidportion at elongation by an applied stress of 3.0 MPa is 5.8% or less.22. The crosslinked rubber composition of claim 21, wherein the volumeof the low density region at elongation by an applied stress of 1.5 MPais 75% or more, and wherein the ratio of a component having aweight-average molecular weight of not less than 1,000,000 in amolecular weight distribution measured by gel permeation chromatographyis 52% by mass or more.
 23. The crosslinked rubber composition of claim21, wherein the ratio of a component having a weight-average molecularweight of not less than 1,000,000 in a molecular weight distributionmeasured by gel permeation chromatography is 52% by mass or more, andwherein the ratio of a component having a weight-average molecularweight of not less than 1,000,000 in a molecular weight distributionmeasured by gel permeation chromatography is 54% by mass or more. 24.The crosslinked rubber composition of claim 17, wherein the ratio of acomponent having a weight-average molecular weight of not less than1,000,000 in a molecular weight distribution measured by gel permeationchromatography is 54% by mass or more.
 25. The crosslinked rubbercomposition of claim 17, wherein the ratio of a component having aweight-average molecular weight of not less than 1,000,000 in amolecular weight distribution measured by gel permeation chromatographyis 56% by mass or more.
 26. The crosslinked rubber composition of claim17, wherein the ratio of a component having a weight-average molecularweight of not less than 1,000,000 in a molecular weight distributionmeasured by gel permeation chromatography is 58% by mass or more. 27.The crosslinked rubber composition of claim 17, wherein the ratio of acomponent having a weight-average molecular weight of not less than1,000,000 in a molecular weight distribution measured by gel permeationchromatography is 60% by mass or more, the rubber component comprising aconjugated diene compound.
 28. The crosslinked rubber composition ofclaim 17, comprising a rubber component comprising a conjugated dienecompound, wherein the volume of the void portion at elongation by anapplied stress of 3.0 MPa is evaluated from a density distribution ofthe crosslinked rubber composition when extended using an X-raycomputerized tomography.
 29. The crosslinked rubber composition of claim17, wherein the volume of the low density region at elongation by anapplied stress of 1.5 MPa is evaluated from a density distribution ofthe crosslinked rubber composition when extended using an X-raycomputerized tomography, and wherein the density distribution of thecrosslinked rubber composition when extended using an X-ray computerizedtomography is evaluated with an evaluation device comprising aphotographing means, the photographing means has an X-ray tube forirradiating an X-ray and a detector for detecting the X-ray andconverting it into an electric signal, the detector has a phosphor forconverting the X-ray into a visible light, and a decay time of thephosphor is 100 ms or less.
 30. The crosslinked rubber composition ofclaim 29, wherein the density distribution of the crosslinked rubbercomposition when extended using an X-ray computerized tomography isevaluated with an evaluation device comprising a photographing means,the photographing means has an X-ray tube for irradiating an X-ray and adetector for detecting the X-ray and converting it into an electricsignal, the detector has a phosphor for converting the X-ray into avisible light, and a decay time of the phosphor is 100 ms or less, andwherein a luminance of the X-ray irradiated from the X-ray tube is 10¹⁰photons/s/mrad²/mm²/0.1% bw or more.
 31. The crosslinked rubbercomposition of claim 30, wherein a luminance of the X-ray irradiatedfrom the X-ray tube is 10¹⁰ photons/s/mrad²/mm²/0.1% bw or more.
 32. Apneumatic tire comprising: bead cores provided on a pair of right andleft bead portions, respectively, a carcass ply extending from a crownportion to the both bead portions through both side wall portions andmoored to the bead cores, an inner liner disposed at an inner side thanthe carcass ply in a direction of a tire diameter, and a tread disposedat an outer side than the carcass ply in a direction of a tire diameterand composed of the crosslinked rubber composition of claim 18.